The invention relates to a pressure indicator of the kind comprising a diaphragm box containing two chambers which are completely separated by a resilient diaphragm, each of said chambers communicating via a connecting channel with an associated one of two fluid sources between which a pressure difference is to be established, wherein a metal plate is fastened to the diaphragm, and an inductive sensor is disposed at one of the walls of the diaphragm box, so that a pressure change in at least one of the chambers causes a change in distance between the metal plate and the sensor and, consequently, an output signal from the sensor.
In electronics, the use of so-called inductive sensors is known. Such a sensor consists essentially of a transistor-oscillator which upon connection to a current source produces a high-frequency electro-magnetic stray field. If a metal object is placed in the magnetic field, eddy currents will flow in the metal material and cause a change in the internal electrical resistance of the sensor. Thus, when the metal object has been advanced to a predetermined distance from the sensor the current in an external signalling circuit will be abruptly reduced, and this sudden change is used as an output signal. Inductive sensors of this kind are used primarily as pulse generators in electronic control devices, but sometimes also for sensing the presence of a metal object.
A pressure indicator of the kind defined in the opening paragraph and provided with an inductive sensor as described above is disclosed in Swedish Pat. No. 7201662-9. Such a pressure indicator has many advantages. Since the diaphragm is not mechanically connected to a bar or the like passing through the wall of the box, there is neither any friction between the movable and stationary parts nor any difference in effective area between the two sides of the diaphragm. The device will therefore be totally balanced and, when indicating small as well as large pressure differences, it operates with great sensitivity and accuracy. Even when both of the pressures to be compared are high, very small differences in pressure can be indicated. Moreover, it is possible to adjust the sensitivity of the device within wide limits by changing the distance between the metal plate and the sensor. Thus, if the metal plate has been originally adjusted so that even a small displacement towards or from the sensor results in an output signal, the sensitivity is great and enables the indication of very small differences in pressure, e.g., 0,1 mm water column. On the other hand, if the device is to react only to larger pressure differences, such a performance can be obtained by increasing or reducing the distance between the metal plate and the sensor, so that the metal plate, in case of a pressure difference between the chambers, must to be displaced a longer distance before it passes the point where the output signal is obtained. Whether the distance between the plate and the sensor should be increased or decreased in such an adjustment process depends, of course, on the side of the diaphragm where the higher pressure is to be expected.
However, a disadvantage of this known pressure indicator is that the diaphragm may easily break in case of a large, possibly unintentional, pressure difference between the two chambers.